The present invention relates to a protector for a molten metal casting stream.
In a continuous casting operation, as shown in FIG. 1, molten metal A supplied from a ladle 10 to a tundish 20 is poured into a continuously vibrated mold 30 to produce a steel strand C. During casting, if a casting stream B is exposed to the air, the molten metal is oxidized, thus causing surface defects and non-metallic inclusion in the cast strand C.
In order to protect the casting stream from oxidation by the air and to improve the quality of the continuously cast strand, there have been proposed various methods and devices, for example, a so-called nozzle submerging method, a gas curtain method and a bellows protecting method.
In the nozzle submerging method, a refractory nozzle for pouring the molten metal is submerged below the meniscus of molten metal in the mold. This is highly effective for preventing oxidation. However, in the continuous casting of a billet having a relatively small cross section, the refractory nozzle easily adheres to the wall of the mold, and this method is thus not practicable, with the exception of the continuous casting of a bloom or a slab.
The second method in which an inert gas curtain is formed around the casting stream is the best in operation, but wastes a great deal of gas, thus resulting in higher cost. Moreover, it is difficult to reduce the oxygen in the atmosphere to less than 1%. Optimum conditions for preventing the casting stream from oxidation is to have the oxygen present be less than 0.1 to 0.4%. Therefore, this method is less effective.
In the third method, a bellows is used for enclosing the casting stream, and an inert gas is supplied into the bellows. This method makes it possible to form an effective non-oxidation atmosphere by supplying sufficient inert gas. However, the bellows, even if made of asbestos, is soon weakened by the radiation of heat from the casting stream. In order to reduce the influence of heat, the diameter of the bellows may be made larger, thus allowing greater capacity for breathing during vibration of the mold which causes alternate expansion and contraction of the bellows. In order to avoid the suction of air, a great deal of gas is needed. Moreover, when an accident requires interruption or stoppage of casting, an emergency runner cannot be used owing to the obstruction created by the bellows being fixed to the bottom wall of the tundish. Therefore, a permanent tundish gate stopper is indispensable, thus resulting in disadvantages in operation as well as in the cost for refractory material. Further, the casting stream and the meniscus of the molded metal cannot be watched since the bellows obstructs the view. Therefore, a permanent level controller for the meniscus of the molded metal is indispensable.
An object of the present invention is to obviate the various defects in the conventional protecting methods and devices, and to provide a casting stream protector which enables reduction of gas consumption, easy mounting and dismounting, improvement of operation, and production of a cast strand having higher quality with few deficiencies.